The present invention relates to a matrix print head particularly of the type used for serially writing, the characters to be composed in each instance of a plurality of dots; the print head is to be comprised basically of an assembly of electromagnets, and the assembly includes a common magnetizable yoke plate, magnetic cores mounted thereto for carrying electromagnetic coils and a permanent magnet to provide magnetic bias to all of the electromagnetically effective parts thus established; moreover, the print head is to include an armature assemby cooperating with but constituting otherwise a separate assembly from the electromagnetic coils and the electromagnetic assembly generally as defined, whereby the armature assemby is to be comprised basically of an armature plate and of a plurality of deflectible arms cooperating with the aforementioned cores and being provided for operating one or more print elements such that in case the arm of an armature is retracted to a first position which obtains from the permanent magnetic bias while through resilient force and through compensation of the permanent magnetic field by electromagnetic energization the stored resilient force of the armature arm causes the print element or elements to be propelled forward.
Matrix print heads of the type to which the invention pertains are usually provided in printers for movement in a reciprocating fashion along a printing platen. Whenever a particular print position is reached and when on account of selective control a particular print element is to be actuated, the magnetic field provided by the permanent magnet and effective in the particular armature arm to which that print element is connected will be compensated or offset through selective control of the respectively associated magnetic core and coil subsystem. Whenever the magnetic bias is effective the respective armature arm, as stated, is retracted and thereby mechanically tension biases the armarture arm is a resilient fashion while upon compensation of this permanent magnetic field the resilient arm relaxes and thereby through this spring relaxation propells the print element in forward direction. Such a print system is generally very effective and is widely used, whereby particularly a very high number of print positions are obtainable per second. Any limitation here results from the total time actually needed for each print element to be propelled forward to provide the dot printing and to be retracted thereafter. These three temporal parameters together limit the print speed.
Basically, one distinguishes between two different systems or type of systems; in one system characters are formed on a serial basis, the characters of a line are printed in sequence by the print head as it travels across the entire line. Alternatively, line printers provide concurrent printing of all, or of at least many characters, on a particular line. In the case of a serial operation the characters are formed individually and all individual dots of a particular character will be produced by the same or by different print elements depending on the contour of that character. In the case of line printing the print element are all arranged in one horizontal line, for example, 66 print elements per head accommodate one line as far as printing characters is concerned, but strictly speaking, all these elements operate in one horizontal dot line and position so that for forming any complete character a relative motion is needed in addition between the print medium and the print head and in a direction transversely to that line in order to yield so to speak the vertical dimension of each character. In case of line printing of this type the matrix print head as such is constructed as and is usually also designated a so-called hammer bank. This hammer bank basically oscillates in the horizontal during operation, covering at least one, possibly two, width dimensions (maximum) of characters to be printed. The displacement of the hammer bank is quite minimal which is, as far as overall speed is concerned, its major advantage but the cost, of course, is considerably higher as compared with a matrix print head which in its entirety has to be moved back and forth along the entire priting platen.
Generally speaking, systems constructed of the type to which the invention pertains have been successfully practiced, and are used, in fact, in large quantities in printers throughout. Nevertheless, it was found that on one hand the amount of magnetic energy to be controlled and handled is quite high and should be reduced for a variety of reasons, while, on the other hand, the particular construction principles involved entail considerable tight tolerances which are prone to lead to problems while on the other hand their observance is a significant cost factor.
In accordance with German Pat. No. 30 17 903 and European patent application (published) No. 9873, it is known to provide a matrix print head of the type referred to above with a disc-shape spring, and to construct the individual arms as radially inwardly extending protrusions of that disc, whereby reinforcement or armature pieces are provided near the tips of these protrusions or arms, in which the print elements, such as needles or wires, are mounted. This type of construction is disadvantaged by the fact that inherently the energy consumption is relatively high, and here reference is particularly made to the energy needed to compensate temporarily the magnetic field provided by the permanent magnet or by an electromagnetic biasing circuit. Particularly, armature arms are guided in slots of a cover plate, and are arranged transversely to these springs. These slots exhibit highly undesired stray flux losses which, in the long run, diminish the effective force of the permanent magnet. A reduction in the force provided by the permanent magnet is directly effective as a slow-down of the retraction of the armature arm whenever the compensated field has dropped and now the armature arm and the actuated print element are supposed to be retarcted from the print position, as fast as possible. In order to offset this aspect it is apparent that in those cases in which armature arms are guided in slots of a cover plate extremely high accuracies have to be observed, i.e. the tolerances of all participating parts have to be extremely tight, which, of course, is possible in principle but quite expensive.
Further, in accordance with the state of the art, it was found that the known arrangement of and with a permanent magnet is disadvantages by the fact that the magnetic flux will inevitably be weakened through field dissapation, i.e. through emanation of undesired stray flux into adjoining components. Also it was found that the known construction, as far as the resilient spring arm mounting is concerned, exhibit disadvantages on configurations.